判断・推量N3
〜わけだ
that's why... / it follows that... (conclusion)
Structure
句子（普通形）+ わけだ
Explanation
Indicates a natural conclusion drawn from the preceding facts, "that's why... / no wonder...".
Examples
彼は毎日練習しているから、上手なわけだ。
He practices every day, so it's no wonder he's good.
なるほど、そういうわけですか。
I see, that explains it.
Note
わけだ states a conclusion or reason, close to ということだ.